STP and Redundant Paths, Is there a method for prioritizing path?

[Anonymous]

I have a Netvanta 1544F that serves as the VLAN switch in the network. It is connected via fiber to eight Netvanta 1534 L2 switches. Each switch is connected point to point with the 1544F. There is an existing wireless network that we want to use as a backup. There is a main access point located in the building with the Netvanta 1544F. There are client access points that connect other buildings where some of the Netvanta 1534's are located.

Where I have a client access point near a 1534, I would like to use the wireless network as a backup to the fiber.

I thought about using port aggregation between the 1544F and the eight 1534's. However, while there is a dedicted fiber between the 1544F and each 1534 (eight connections at the 1544F), there is only one main access point. I can only connect that access point to one copper port on the 1544F. I don't believe that the 1544F will allow me to aggregate that single port eight times with eight different ports.

The fiber provides enough bandwidth for the network. The wireless is simply a backup. If a fiber segment goes down, the wireless should serve as the path to the switch.

I believe that the alternative to port aggregation is to allow STP to disable the ports connected to the wireless clients on the 1534's if the fiber path is intact. The wireless is slower than the fiber. We don't want STP to select the wireless path if the fiber is intact. Is there a way to prioritize paths for STP?

Another method might be to use probes to shutdown the wireless port on the 1534 if the fiber to the 1544 is intact. I've not investigated this in depth. I'm open to suggestions.

[Anonymous · Accepted solution]

@bcrinehart - You are correct in your thoughts that we cannot aggregate a single port eight times over with eight different ports.

In your scenario, STP should be a valid option. You can use the "spanning-tree priority 0" command on the switch you would like to act as the 'root bridge'. The 'root bridge' will have all of its ports in a forwarding state. In your case, I would recommend that you use this command on the 1544F.  Once this is done, the 1534s will then elect which port will act as the 'root port'. 'Root ports' are those ports that are determined to be closest to the 'root bridge' by STP. Network administrators can use the "spanning-tree cost" command to manipulate STP into choosing a preferred path on non-root bridges (in this case, the 1534s). This command is set at the switchport level. The lower the cost, the more preferred the path is. Also, if the speed the access port is set or negotiating to on the 1534 to 100 Mbps, its possible that STP will choose the fiber link as the preferred path without any modification to the "spanning-tree cost" since it will automatically prefer the faster link.

Let us know if you have any further questions.

Thanks,

Noor

[Anonymous]

- I went ahead and flagged this post as “Assumed Answered.”  If any of the responses on this thread assisted you, please mark them as either Correct or Helpful answers with the applicable buttons.  This will make them visible and help other members of the community find solutions more easily.  If you still need assistance, I would be more than happy to continue working with you on this - just let me know in a reply.

Thanks,

Noor

[Anonymous]

I'm writing to comment on the response from Noor. I implemented STP as suggested and it works. We accidently discovered that STP works when a contractor cut one of the fiber cables with a cutting torch. We discovered the broken cable when we lost communications to some locations that were out in the plant. The locations that were served by the wireless backup remained connected. STP discovered that the fiber path was lost and enabled the wireless path between the 1534 and the 1544F.